On-the-fly cryptography packages such as TruCrypt allow the creation of a hidden, encrypted volume (containing files and directories) on a hard disk. Users typically create Alice, a standard encrypted file system on a laptop, protected by a password. A second (deniable) file system can be created inside this encrypted volume, with access controlled by a second password.

However, Windows Vista Microsoft Word and Google Desktop all compromise the deniability of a TrueCrypt DFS steganography, according to a team including crypto guru Bruce Schneier and researchers from the University of Washington.

The problem is that applications such as Word or Google Desktop leak information outside of the deniable volume - for example, in lists of recently changed documents or snapshots captured by Google Desktop, providing the "enhanced search" option is enabled. Potentially worse still, Word auto-saves the contents of encrypted files as plain text to open portions of a hard disk. Although the research was carried out using TrueCrypt version 5.1a, the issue could apply to other non-full disk encryption software.

"Our results suggest any DFS will not only have to encrypt and hide data — as file systems like TrueCrypt do — but must also erase any traces of that data left by the operating system through normal operation," the researchers conclude.

The DFS approach is gaining increased attention as a means to hide sensitive information from laptop border searches, prompting the interest of the researchers, who suggest approaches for improving the technology. Even if these suggested changes are made, crypto boffins caution against blind trust in the effectiveness of such systems.

TrueCrypt version 6, released at the start of July, introduces new features such as the ability to create deniable operating systems which were beyond the scope of study.

Schneier, CTO of BT Counterpane, toldDark Reading that although this version will "definitely close some of the leakages, but it's unlikely that it closed all of them".

The team's research paper, Defeating Encrypted and Deniable File Systems: TrueCrypt v5.1a and the Case of the Tattling OS and Applications, is due to be presented at the USENIX Workshop on Hot Topics in Security (HotSec 2008) conference in San Jose at end of this month. ®